This invention relates to digital transmission networks and, more particularly, to apparatus for distinguishing between failures occurring in a digital transmission network.
A hallmark of an efficient digital transmission network is the achievement of a quality of service deemed acceptable by the users of the network. One way to express the quality of service is in terms of the number of failures occurring at various levels of the network. An illustrative network is the Bell System digital transmission network wherein the various levels of the digital hierarchy of the network are usually designated in relation to the bit rate of the digital signal there present. For example, a 1.544 megabits per second (Mb/s) signal is designated a DS-1 digital signal. Accordingly, facilities including transmission link for transmitting and receiving the DS-1 signal are usually designated as being at the DS-1 level. As to the DS-1 signal, a typical network includes an analog-to-digital transmitter encoder to quantize a speech sample from a channel and to generate an eight-bit digital character. Digital characters, whether representing data, quantized speech, or other digital signal, from up to 24 channels are then multiplexed by a channel bank into a 193-bit frame. The frame usually includes a one-bit framing signal and 24 time slots, a time slot for a channel. Typically, the framing bit is caused to alternate between a logic zero in one frame and a logic one in the next adjacent frame. It is the frame which is thereafter transmitted as the DS-1 signal. At a receiver, the frame is advantageously demultiplexed through a receiving channel bank so that each character can be extended to a respective output channel. Of course, other signal levels may also be used in the network. For example, in the Bell System network a 6.312 Mb/s signal is called a DS-2 signal, while a 44.736 Mb/s signal is called a DS-3 signal. Clearly, there being different signal levels, there exists a need to transform signals from one level to another level. Exemplary apparatus for transforming up to four DS-1 signals to or from a single DS-2 signal is an M12 digital multiplex. Still other apparatus is a M13 digital multiplex for transforming up to 28 DS-1 signals to or from a single DS-3 signal.
It should come as no surprise that providing service of a specified quality, perhaps more especially when provided under variable conditions including network growth and unavoidable equipment failure, requires a maintenance plan. Under a maintenance plan the quality of service can be expressed in many ways. One way has been previously mentioned. Another way is in terms of the time required to detect and isolate a failure. It is well known to include failure detection apparatus at each level of a digital transmission network. For example, it is common that alarm control circuitry at a receiving channel bank continuously monitor an incoming digital signal. Occasionally there may be a failure in the digital signal, as for example a loss of frame. Responsive to the detection of the out-of-frame condition, an alarm is normally excited at the receiving channel bank, e.g., it is common that a red lamp be lighted to display a steady condition. Also, the receiving channel bank usually generates a special reverse signal for transmission in the reverse direction. A common reverse signal includes a bit stream having one or more bit positions of each character set to a fixed binary state. The set bits are detected by alarm circuitry at the transmitting channel bank and, responsive thereto, an alarm is excited thereat, e.g., a steady yellow lamp may be lighted. Thusly, if a failure is detected in one direction of transmission, alarms are usually excited at both ends of the network.
For purposes of exposition, we conceptually bifurcate the network between low level equipment, such as a channel bank and its incoming/outgoing transmission link, for processing digital signals at or below the bifurcation signal rate, e.g., the DS-1 rate, and high level equipment, hereinafter called the digital hierarchy, for processing digital signals above the bifurcation rate. If a failure occurs in the digital hierarchy, it will typically propagate downward to a low signal level. As a result, a failure in the digital hierarchy will usually excite alarms at many channel banks. Unfortunately, the conventional alarm is such that a craftsperson who is situated at an alarmed channel bank is unable to distinguish between a digital heirarchy failure and a channel bank failure. Accordingly, the craftsperson typically commences to search for the source of the failure, usually by testing various portions of the channel bank and its incoming/outgoing transmission link. Finding no failure in the channel bank, the craftsperson commonly turns to the hierarchy. Of course, the search procedure may be reversed. Notwithstanding, the expenditure of time in distinguishing between the hierarchy and the channel bank while isolating the failure may lead to degradation in the quality of service.
Accordingly, a broad object of our invention is to provide improved apparatus for distinguishing between failures in a digital transmission network.
It is well known that digital networks are rapidly expanding. As a consequence, vast numbers of network equipments have been built and are being used. To directly modify existing equipment could well be economically unwise.
Accordingly, another object of our invention is to provide distinguishing apparatus without substantial modification to the network.